My mom developed a chemical reaction to silver & gold. This was the good stuff - sterling silver & 24-carat gold. So I went to the craft store & purchased a spray coating & tried it, ( 1st on the silver!), and the silver didn't turn black & neither did her neck. She loved it. She didn't put it on everything but items she wore regularly and inexpensive jewelry.
Yes, I now spray all my inexpensive jewelry so that it retains its patina. ( You can even use 'Rustoleum'; they have clear gloss & a clear matte finishes.) This little trick keeps my 'gold' & 'silver' shining. Just make sure that is in a position so the charm will move freely. It's so pretty!